1. Field of the Invention
The present invention relates to an articulated robot. More particularly, it relates to an articulated robot utilizing a gear mechanism for transmitting rotation of a servomotor to a second arm.
2. Discussion of the Prior Art
In a conventional articulated robot, rotation of a servomotor is transmitted to the second arm through various mechanisms such as a parallel link mechanism, a belt drive mechanism, a direct drive mechanism and the like. A robot using the belt drive mechanism has a problem in that the stiffness is generally low. A robot having the direct drive mechanism has a problem in that a heavy servomotor must be attached to the forward end of the first arm for swinging the second arm. Since the heavy servomotor acts as a load on the first arm, which produces a gravity and an inertia moment, the load capacity of the robot is relatively small. For the reason above, in an articulated robot which is required to have a large load capacity, a parallel link mechanism is used for transmitting the rotational torque of the servomotor to the second arm, as disclosed in the Japanese laid open utility model publication 1-121683.
As shown in FIG. 8, the robot is provided with a swivel base 12 supported on a support base 10 for rotation about a vertical axis, a first arm 14 whose lower end is pivoted on the swivel base 12 for swing movement about an axis perpendicular to the vertical axis and a second arm 15 pivoted on the forward end of the first arm 14 for a swing movement about an axis parallel to the swing axis of the first arm 14. The second arm 15 is provided with a wrist portion 19 at its forward end. The robot is further provided with a first axis servomotor 13 for rotating the swivel base 12 with respect to the support base 10, a second axis servomotor 16 for swinging the first arm 14 with respect to the swivel base 12 and a third axis servomotor (not shown) for swinging the second arm 15 with respect to the first arm 14. The rotational torque of the third axis servomotor is transmitted to the second arm 15 through a parallel link mechanism 18.
In the parallel link robot, the posture of the second arm 15 does not change when only the first arm 14 is driven to swing from it neutral angle shown in FIG. 9(a) to a predetermined angle shown in FIG. 9(b). With this feature, the parallel link robot has an advantage that the angular position of the arms are easily controlled. Also, by employing the parallel link mechanism, the third axis motor swinging the second arm 15 can be mounted on the swivel base 15, thereby decreasing the load applied to the first arm 14.
However, the parallel link robot has a disadvantage that the wrist portion 19 cannot be moved to the back side of the robot and the upper side of the robot, because the working area of the robot is limited in the front side of the robot, as shown by the broken line 1 in FIG. 8. This problem occurs because of interference between the link mechanism 18 and the swivel base 12.